While reaching for a Granny Smith, the wall of sound rattled me. I dropped the apple, pulse racing: I had no idea such massive choral noises could emanate from inside a grocery store.

I was at the People's Food Co-op in Southeast Portland, and a hearty group of amateur singers were belting out 18th- and 19th-century choral music in a tiny room just above the first-floor market. Shoppers went about their business, with one casting a curious eye at the distracted cashier. "Some choir rehearsal," she mumbled.

No no, not a rehearsal. A singing.

[audio:2007/10/17/DS_20117.mp3] (Scroll down for more live audio.)

And not exactly a choir, if any formal definition of the term would be applied. The assembled singers come together sporadically: some several times a month, others less often. Their commonality? A love of Sacred Harp singing.

An early American form of sacred choral music based on a widely distributed hymn book first published in 1844, Sacred Harp singing is fast becoming the drag-yourself-away-from-YouTube activity of choice. Regional conventions and community singings are springing up in even the hippest, most doggedly non-dogmatic urban communities. Like Portland.

"Sacred Harp has all the upsides of a religious community, without the exclusionary aspects of religion," says 20-year-old Chris Cotter, a Reed College student active in Sacred Harp both in Portland and nationally. He says the Sacred Harp's weighty Protestant texts are interpreted freely by participants: "The singers find their own personal resonance with the music."

The group is nothing if not democratic. At the Friday night Food Co-op singing, singers ambled in early and late, many with copies of the Sacred Harp hymnal tucked under an arm, though rental copies ($5) were available for newbies. Words like "family" and "community" flowed freely. A guy in biking shorts named Dan started the evening off: "We're here to sing for the joy of it," he said, "and not to worry about whether we're 'on' or not."

In fact, several of the singers were blissfully far from "on," but that didn't impede their enthusiasm (or volume). It's precisely this lack of thundering authority and the all-embracing sweep of the music that attracted singer Jessica Beer, a key organizer of this weekend's Sacred Harp singing convention at the Mississippi Ballroom.

"We connect with one another on a more base level," she says. "We also leave politics and religion at the door." She likes to focus instead on the "penetrating sound that gets under your skin…that primal scream of music" that is Sacred Harp singing at its best. Just alert shoppers next time.